On one particular project, Boart Longyear mobilized underground coring rigs to a client’s mine in the US where the mine was faced with a big water problem that was making the underground exploration coring impossible to complete. The mine had two shafts located at different levels, one above the other. The deeper shaft had pumps installed and was ready to pump water to the surface. However, the upper shaft had flooded with water – where there were no pumps.

Typical underground coring setup.

Rather than attempt to install additional pumps in the flooded upper shaft to remove the water, Boart Longyear moved their underground coring rigs down to the lower shaft and drilled a fan-shaped series of up-holes to the flooded shaft above. The idea worked, effectively using gravity to drain the water from the upper shaft to the lower shaft where the pumps were then able to move the excess water to the surface. The mine was able to continue operations in the upper shaft and Boart Longyear was able to start the diamond coring exploration drilling.

On another project, the mine was thought to be dry. After the underground coring began, the Boart Longyear crew ended up finding water – a lot of it, quite unexpectedly. The crew was able to come up with a plan to manage the water so the exploration drilling could be completed. After pressure grouting in a cement collar casing, a blow-out- preventer (BOP – used to seal, control, and monitor water to prevent blow outs and the uncontrolled release of high-pressure water) and rock pack were installed successfully, shutting the water in. Most instances where that amount of water is encountered play out quite differently. Everyone has experienced, or knows someone who has, a situation where a high volume of water blows all your tooling out of the hole, resulting in a complicated mess.

Other examples of unexpected challenges include creative solutions using Boart Longyear’s own engineered underground rigs on the surface. The Drilling Services Underground Coring division mobilized rigs to a mine site where a delay in underground diamond coring exploration had the underground rigs idle on the surface. Someone saw the rigs waiting and asked if the idle underground rigs could drill flat-angled holes on the surface. Accepting the challenge, the underground coring rigs have been drilling these low, flat-angled boreholes ever since.

The mine site has a limited surface disturbance permit – which means that to reach the geological formations outside the area of their disturbance permit, Boart Longyear’s Underground Coring division is operating underground rigs on the surface to drill low-angled and flat holes that surface rigs often can’t reach because of their truck-mounted tall masts.

The underground rigs can drill at low angles from -40 degrees down to flat. They drill underneath the area where the mining client isn’t allowed to disturb the surface. This means exploration coring can follow ore veins without disturbing the surface and drill beneath the area outside of the issued disturbance permit.

The boreholes are 2600 ft to 2700 ft (792.48 m to 822.96 m) at a -40 to -12 degree angle. Boart Longyear has the tooling and the expertise for that type of drilling but was unable to accomplish the job with surface rigs because when set up flat, you’re going to be 12 ft (3.66 m) off the ground because the mast is way up on a truck carrier. With the underground rigs, a work platform or deck was constructed, which enabled the team to work right up close to them.

The Boart Longyear Drilling Services Underground Coring division in the US runs all Boart Longyear-engineered and manufactured LM™ and MDR rigs. The LM series of rigs feature easily interchangeable components, allowing for space constraints without sacrificing depth requirements. These modular coring rigs offer easier access in tight underground spaces and mobility for easier moves from hole to hole or shaft to shaft. Boart Longyear’s patented tooling offers safer and more efficient up-holes at any angle.

The underground coring rigs in the Drilling Services’ fleet are modified and customized, as compared to the commercially available Boart Longyear underground coring rigs. For example, the LM™90 is souped up with a 150-horsepower engine instead of the typical 90-horsepower engine and includes additional modifications for increased depth capacity and core size.

The Drilling Services Underground Coring division has the experience and technical expertise to drill dewatering galleries, geotechnical borings, tunnel investigations, grouting services, and utility borings. The division also has experience with overcore drilling to measure in-situ stress. Overcore drilling involves drilling a hole to depth with a small diameter EX- or AX-sized bit, placing a probe to measure strain response data, and then using a special resin to glue the probe in place. After drilling out a larger diameter core to retrieve the probe, the overcore sample and probe are used for stress testing the rock formation to plan for ground support. This method is often used in both horizontal tunnels and vertical shafts to measure the amount of stress the rock can take.

Another unique application for underground rigs and crews is drilling on the surface into high walls for dewatering purposes. Flat to close-to-horizontal boreholes – from 10 degrees up to 10 degrees down – can be used to create self-draining dewatering boreholes in high walls. Pressure grouting in a cement collar casing keeps the hole from collapsing and can be used with a BOP valve in case the mining client wants to shut the water in the rock formation.

To maintain the integrity of the hole, installing slotted polyvinyl chloride (PVC) keeps the hole from collapsing. If you go through a broken zone, when water starts flowing, the hole can bridge off after a couple of months. The PVC keeps the hole open and the water running.

The benefit of using underground rigs for this type of drilling is the difference between large rotary rigs that produce chip samples and use lower rotation per minute (rpm) and higher torque whereas a coring rig operates at higher rpms and less torque. Chuck-drive underground coring rigs drill slower, straighter, and deeper and collect core for further analysis.

Geologists can use core samples from horizontal drilling into a high wall to look for fractures, faults, and oxidation (to identify water-bearing zones). They can also be identified in chip samples using rotary drilling rigs, but it’s more difficult to see them that way. A core sample provides a better physical view of the rock formation. Drilling high walls with underground coring rigs accomplishes two goals with one borehole – dewatering and obtaining core samples for assaying for fractures, faults, and oxidation.

LM™90 with customized feed frame. Expert drillers from left to right: John Sanchez, Jimmie Maggard, Marcus Woody, and Sal Renteria.

Whether it’s an underground coring project or a surface application for underground coring rigs, the drilling crews are the real heroes at Boart Longyear. They have wide-reaching (global) resources to find unique and inventive solutions for complex, technical, and sometimes immense drilling challenges and they’re not afraid to ask questions.

More than just a can-do attitude, great work ethic, diverse skills, vast experience, technical knowledge, and a commitment to safety and each customer’s success, they get the job done. Drillers, driller assistants, welders, mechanics, supervisors, and other support personnel travel to work in remote locations, in all kinds of challenging weather, climates, and altitudes, all while spending extended time away from their families. The work is strenuous, involves extensive training, and requires their full attention for safety’s sake.

These real heroes all contributed to Boart Longyear’s latest safety achievement of 10 million man-hours and one year of being lost-time injury-free. A big congratulations to all the drilling crews and support personnel on this company-wide accomplishment! Denis Despres, Chief Operating Officer stated, "On a site level, we value the everyday procedures from standardized pre-start information meetings that focus on daily job tasks and associated hazards and risks, our online data management system, including the system’s mobile app, and the daily team and individual-based field level risk assessments. At Boart Longyear, it is everyone’s responsibility to work safely."

Originally published in Issue 11 of Coring Magazine, November 2019